The present invention concerns voltage regulators, and more particularly, to a dual voltage voltage regulator with foldback current limiting wherein the threshold for initiating current limiting is maintained at approximately the same output current for each of the output voltages.
Voltage regulators, which use a controllable series impedance device for maintaining a regulated output voltage coupled to a load, are susceptible to damage if a short circuit or other fault is applied to the output terminals of the regulator. Such damage often is caused by excessive thermal dissipation of the series impedance device or by greatly exceeding the current rating of the series device. For this reason, it is common to provide overload protection to prevent such damage to the regulator.
One type of overload protection is current limiting in what is known as a "foldback" voltage regulator, such as is disclosed in U.S. Pat. No. 3,445,751 of Easter. Such a regulator provides output voltage regulation for a changing load until an overload current threshold is reached. For load currents above this threshold, the available output current decreases as the load increases, with a corresponding decrease in the output voltage. The short-circuit current can be adjusted to be but a small fraction of the full load current, thus minimizing the dissipation in the series pass transistor. The voltage regulator of the present invention is such a "foldback" voltage regulator.
Some applications require a voltage regulator which is capable of providing multiple output voltages. Accordingly, it is desirable to provide a multiple voltage voltage regulator having current limiting overload protection for both output voltage settings.